To prevent the ozone layer depletion, global warming, and the like, natural refrigerant such as NH3 or CO2 has been reviewed as refrigerant in a refrigeration apparatus used for room air conditioning and refrigerating food products. Thus, refrigeration apparatuses using NH3, with high cooling performance and toxicity, as a primary refrigerant and using CO2, with no toxicity or smell, as a secondary refrigerant have been widely used.
In the refrigeration apparatus, a primary refrigerant circuit and a secondary refrigerant circuit are connected to each other through a cascade condenser. Heat exchange between the NH3 refrigerant and the CO2 refrigerant takes place in the cascade condenser. The CO2 refrigerant cooled and liquefied with the NH3 refrigerant is sent to a cooling device disposed in the freezer, and cools air in the freezer through a heat transmitting pipe disposed in the cooling device. The CO2 refrigerant partially vaporized therein returns to the cascade condenser through the secondary refrigerant circuit, to be cooled and liquefied again in the cascade condenser.
Frost attaches to a heat exchanger pipe disposed in the cooling device while the refrigeration apparatus is under operation, and thus the heat transmission efficiency degrades. Thus, the operation of the refrigeration apparatus needs to be periodically stopped, to perform defrosting.
Conventional defrosting methods for the heat exchanger pipe disposed in the cooling device include a method of spraying water onto the heat exchanger pipe, a method of heating the heat exchanger pipe with an electric heater, and the like. The defrosting by spraying water ends up producing a new source of frost, and the heating by the electric heater is against an attempt to save power because valuable power is wasted. In particular, the defrosting by spraying water requires a tank with a large capacity and water supply and discharge pipes with a large diameter, and thus increases plant construction cost.
Patent Documents 1 and 2 disclose a defrost system for the refrigeration apparatus described above. A defrost system disclosed in Patent Document 1 is provided with a heat exchanger unit which vaporizes the CO2 refrigerant with heat produced in the NH3 refrigerant, and achieves the defrosting by permitting CO2 hot gas generated in the heat exchanger unit to circulate in the heat exchanger pipe in the cooling device.
A defrost system disclosed in Patent Document 2 is provided with a heat exchanger unit which heats the CO2 refrigerant with cooling water that has absorbed exhaust heat from the NH3 refrigerant, and achieves the defrosting by permitting the heated CO2 refrigerant to circulate in the heat exchanger pipe in the cooling device.
Patent Document 3 discloses a method of providing a heating tube in the cooling device separately and independently from a cooling tube, and melts and removes the frost attached to the cooling tube by permitting warm water or warm brine to flow in the heating tube at the time of a defrosting operation.